1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, and a digital copying machine, and an optical multi-beam scanning device capable of being utilized for such an image forming apparatus.
2. Description of the Related Art
Conventionally, in a method for synthesizing a plurality of beam optical paths, polarization characteristic is utilized to prevent a decrease in optical efficiency as follows (see Japanese Patent Application Laid-Open No. 2000-035546).
When two beams are synthesized, one beam passes through a half-wave retardation plate having an angle of 45 degrees with respect to a polarization direction of an incident light in a major axis and the polarization directions of two beams are arranged to have an angle of 90 degrees each other. Then, the two beams are synthesized with a polarization beam splitter surface. The beams pass through a quarter-wave retardation plate which emits circularly polarized light so that transmittance does not differ at a time of passing through an optical system due to the difference of polarization directions between two beams.
However, a conventional method has a number of limitations at an angle around the beam moving direction of a semiconductor laser junction surface and at an installation angle of a polarizing beam splitter.
For example, an optical scanning device includes a first lens and a cylinder lens disposed between a deflector (polygon mirror) and the first lens. The first lens reduces a divergence angle of a beam or changes the beam to a parallel and moderate converging light beam so that a divergent light radiated from a semiconductor laser is substantially parallel light beam. The cylinder lens focuses the beam emitted from the first lens in a sub-scanning direction nearby the optical scanning device. The optical scanning device deflects the beam passing through the cylinder lens by a deflector, focuses the deflected beam on a scanned surface by an image formation optical system, and scans the focused image at a uniform rate. In this case, the reason why the beam is condensed on the deflector in the sub-scanning direction is that the image formation optical system arranges a reflection surface and the scanned surface of the deflector in conjugate relation, thereby preventing an unevenness of the beam position on the image surface in the sub-scanning direction due to a surface inclination of a deflection plane.
In the same image formation optical system, a beam diameter on the image surface (scanned surface) in a main scanning direction is inversely proportional to a beam diameter on the deflector in the main scanning direction. A beam diameter in the sub-scanning direction is proportional to a beam diameter on the deflector in the sub-scanning direction. A beam diameter on the deflector in the sub-scanning direction is inversely proportional to a beam diameter on the cylinder lens in the sub-scanning direction and proportional to the distance between the cylinder lens and the deflector. The beam diameter on the cylinder lens in the sub-scanning direction is proportional to a beam diameter on the deflector in the main scanning direction.
In this case, defining a polarization direction in a predetermined direction determines a direction of the semiconductor laser junction surface to define a radiation angle in the main and sub-scanning directions. Fixing the first lens position determines a beam diameter on the first lens in the main scanning direction from the beam diameter on the deflector in the main scanning direction so that a focal distance of the first lens is determined. Defining the above-mentioned values substantially determines a beam diameter on the cylinder lens in the sub-scanning direction as an emitted beam from the first lens is close to a parallel ray so that the cylinder lens position is determined from the beam diameter on the image surface in the sub-scanning direction.
Thus, in an image formation optical system, fixing a polarization direction and then a beam diameter on an image surface determines the cylinder lens position. This increases an implementation limitation.
On the other hand, when an LD array is used, an LD array junction surface angle is adjusted to make a beam pitch on a photoconductor in a predetermined distance. In this case, a polarization direction is at an arbitrary angle. In order to maintain high optical efficiency by a polarizing beam splitter, it is necessary to determine an angle of the polarizing beam splitter so that a beam from one LD array in a plurality of LD arrays is incident as a P wave or an S wave against a reflection surface of the polarizing beam splitter.
When the angle of the polarizing beam splitter is large against the sub-scanning direction, it is necessary to incline the beam which is incident on the polarizing beam splitter with respect to the sub-scanning direction. As a result, an optical unit tends to be large in thickness in the sub-scanning direction. For example, when a polarization direction is inclined by 45 degrees with respect to a main scanning direction, each element of an optical system is disposed on the straight line which is inclined by 45 degrees from a scanning plane until the beam reflected on the polarization beam splitter surface is incident on the polarizing beam splitter. As a result, an optical unit tends to be large in thickness in the sub-scanning direction.